C Storage Classes

auto: local, automatic storage
register: fast access, local
static: local scope, static lifetime
extern: global scope, defined elsewhere

Storage classes determine the scope, lifetime, and storage location of variables.

1. auto (Automatic)

void func() {
    auto int x = 10;  // local variable (auto is default)
}

void func() {
    register int counter;  // suggest storing in register
}

void func() {
    static int count = 0;  // retains value between calls
    count++;
}

// file1.c
int global_var = 100;

// file2.c
extern int global_var;  // declaration